1) Field of the Invention
This invention relates generally to fabrication of semiconductor devices and more particularly to the fabrication of buried contacts.
2) Description of the Prior Art
Buried contacts are well known in the fabrication of semiconductor devices. However, conventional processes have the problem of the buried contact trench that causes electrical shorts in the buried contact. Buried contact trenches occur when photoresist masks are misaligned and trenches are etched into the substrate in the buried contact region.
A conventional sequence of steps for forming a buried contact follows. A silicon substrate is provided having isolation structures and a gate oxide. A thin layer of doped polysilicon is formed over the gate oxide. Photoresist is applied, and openings are patterned defining areas for the buried contacts. Using reactive ion etching (RIE), the polysilicon and gate oxide are removed in the areas of the openings in the photoresist. A buried contact region is formed by ion implantation. A second polysilicon layer is formed. This layer is typically doped in-situ. Next, a second photoresist mask is patterned defining a polysilicon gate region and the buried contact region. A second opening is formed in the polysilicon and gate oxide in the area between the polysilicon gate and the buried contact not masked by photoresist, using REI. A lightly doped drain (LDD) is formed by ion implantation through the second opening.
Mask mis-alignment in either direction will have a significant detrimental effect. If the second mask is misaligned so as to expose a portion of the substrate where the gate oxide has been removed by the previous etch, a troublesome buried contact trench will be formed in the buried contact. A buried contact trench has the potential to cause an open or high resistance contact, especially in shallow implanted devices. If the second mask is misaligned so as to leave a gap between the second opening and the buried contact, an un-doped area will remain between the buried contact and the LDD causing parasitic resistance.
One approach to overcoming the buried contact trench is disclosed by Liaw et al. (U.S. Pat. No. 5,721,146), whereby a trench is formed in the substrate through the first opening, allowing for formation of a much deeper buried contact. Liaw's invention is less suceptable to the effects of a buried contact trench, but it does not prevent formation of a buried contact trench. Also, it does little to mitigate the effects of an un-doped contact area gap caused by misalignment of the second mask away from the first mask.
Another approach to overcome the detrimental effects of mask misalignment in buried contact formation is disclosed by Huang (U.S. Pat. No. 5,607,881), whereby, following LDD implantation, an additional photoresist mask is patterned with an opening that has been sized up from the original buried contact mask. An extra ion implantation is performed which either underlies the buried contact trench or penetrates the previously un-doped contact area gap. Again, this process does not prevent buried contact trench formation. Also, an expensive photolithography step is added to the process.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering
U.S. Patents
______________________________________ 5,705,437 Wu et al. 5,626,932 Lee et al. 5,607,881 Huang 5,721,146 Liaw et al. 5,668,051 Chen et al. 5,258,096 Sandhu et al. ______________________________________